FIG. 21 shows a conventional slot array antenna which comprises a plurality of slots b equidistantly formed within a plate of rectangular waveguide a. The electromagnetic waves propagate within the rectangular waveguide a in the mode TE.sub.10. The electrical power radiates from each slot b. FIG. 22 shows the power density distribution within the waveguide.
FIG. 23 shows another conventional antenna having a circular waveguide. The electrical power is fed from a power feeder opening 11 formed within the center of a circular plate 13 and propagates within a space S formed by means of a pair of metallic circular plates 12 and 13 and an annular side plate 14. Slots 12a' are arranged in a coaxial array upon the plate 12, each slot 12a' having a cross shape configuration of the same dimension. The power radiates from each slot 12a'. Residual power remaining within the circular waveguide is absorbed by means of a terminal resistor 16. A circular polarized wave generator is attached to a circular power feeder waveguide 18' as a power feeder means for radiating the power under equiphase conditions.
FIGS. 24 and 25 show another conventional antenna having a different configuration and arrangement of the slots. The electrical power is fed to the circular waveguide through means of a power feeder 18 of a coaxial cable. Within the antenna, as shown in FIG. 25, the direction of a particular slot 12a is perpendicular to that of an adjacent slot 12a so as to form a pair of slots. Both slots of each pair are disposed at a distance of one fourth (.lambda.g/4) of the wavelength .lambda.g in the radial direction of the plate 12. The resultant electric field of the wave radiated from the pair of slots 12a has the configuration of a circularly polarized wave. The pairs of slots 12a are spirally disposed upon the plate 12 as is schematically illustrated along a dash-dot line DS so that the wave generated by means of the entire array of slots 12a comprises the circularly polarized wave.
FIG. 28 shows still another conventional antenna in which the waveguide space is vertically divided into a lower waveguide space S1 and an upper waveguide space S2 by means of an intermediate horizontally disposed metal plate 15. The terminal resistor 16 is provided at the center of the space S2 or along the axis thereof. The electrical power fed from the power feeder opening 11 propagates within the waveguide space so as to pass through the lower space S1, an annular gap D defined between the side plate 14 and the intermediate plate 15, and the upper space S2. The power of the antenna radiates from the slots 12a in an equiphase mode.
However, there are problems in such conventional antennas as follows.
In connection with the antenna shown in FIG. 21, each slot b has the same coupling rate, which represents the rate of power radiating from each slot b, as the others. Consequently, the power density within the waveguide a exponentially decreases as shown in the graph of FIG. 22. As a result, the amplitude distribution within the antenna is irregular so that the side lobe becomes large and the antenna gain is reduced.
In the circular waveguide, the internal electromagnetic field density decreases with the distance r from the power feeder opening 11 as shown by means of the curve Po of FIG. 26. The internal electromagnetic fields couple with the power radiation slots 12a so as to be radiated from the slots 12a as an electromagnetic wave in free space. A curve P1 of FIG. 26 represents the radiation characteristics thereof. Thus, as shown in FIG. 27, the aperture power distribution is irregular, so that the aperture efficiency is decreased. In addition, the slots disposed adjacent the resonance wavelength affect the power feeder so as to produce a higher order mode.
In connection with the antenna shown in FIG. 28, the power is guided to a central portion within the upper space S2 by means of the side plate 14. Consequently, the power density has a comparatively flat characteristic as shown in FIG. 29, and the power distribution obtained is as shown in FIG. 30. However, the power fed within the waveguide is reflected at the power feeding portion and by means of the side plate 14.
FIG. 31 shows an antenna in which a conical matching member 17 is mounted within the upper end of the power feeder 18 and the side plate 14 is formed such that the inside wall thereof has a V-shaped cross section, thereby preventing the power from reflecting. However, in such an antenna, it is difficult to manufacture the waveguide and manufacturing costs increase.